[Istio] Revert Istio metric type to object

[constanca-m]

What does this PR do?

Reverts the metric type to object again. The type for the metric as it is now was introduced in #6889.

This is related to the work mentioned in elastic/kibana#155004.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.

[elasticmachine]

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• Start Time: 2023-08-30T06:41:33.110+0000

• Duration: 16 min 31 sec

Test stats 🧪

Test	Results
Failed	0
Passed	14
Skipped	0
Total	14

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (1/1)	💚
Files	100.0% (3/3)	💚 3.362
Classes	100.0% (3/3)	💚 3.362
Methods	90.0% (27/30)	👎 -1.828
Lines	97.472% (347/356)	👍 9.163
Conditionals	100.0% (0/0)	💚

[elasticmachine]

Package istio - 0.4.1 containing this change is available at https://epr.elastic.co/search?package=istio